Wheel bearing apparatus is generally known that can support a wheel of vehicle with respect to a suspension apparatus. The wheel bearing apparatus also incorporates a rotational speed detecting apparatus to detect a rotation speed of a wheel of vehicle to control the anti-lock braking system (ABS). Such a bearing apparatus generally includes a sealing apparatus arranged between the inner and outer members, rotating relative to each other, via sandwiched rolling elements. The sealing apparatus is integrally formed with a magnetic encoder with magnetic poles alternately arranged along its circumference. A rotational speed sensor to detect change of magnetic poles of the magnetic encoder caused by the rotation of a wheel is adapted to be mounted on a knuckle. Thus, forming part of a suspension apparatus after the wheel bearing apparatus has been mounted on the knuckle.
A structure shown in FIG. 24 is known as one example of a wheel bearing apparatus. This wheel bearing apparatus includes an outer member 100, an inner member 101, and a plurality of balls 102 contained between the outer member 100 and the inner member 101. The inner member 101 has a wheel hub 103 and an inner ring 104 fit onto the wheel hub 103.
The outer member 100 has on its outer circumference an integrally formed body mounting flange 100b. The outer member inner circumference includes double row outer raceway surfaces 100a, 100a. A sensor 113 is secured on the knuckle 115 via a bolt 116.
The wheel hub 103 is integrally formed with a wheel mounting flange 105 for mounting a wheel (not shown) on one end. The wheel hub also includes an inner raceway surface 103a. A cylindrical portion 103b axially extends from the inner raceway surface 103a. The inner ring 104, formed with an inner raceway surface 104a on its outer circumference is axially immovably secured to the cylindrical portion 103b by a caulked portion 103c formed by plastically deforming the end of the cylindrical portion 103b. 
A sealing ring 106 is fit into the outer end of the outer member 100. A lip of the sealing ring 106 slidably contacts with a base portion 105a of the wheel mounting flange 105. On the other hand, an encoder 107 is mounted on the inner end outer circumference of the inner ring 104. The encoder 107 has an annular supporting member 108 with an L-shaped cross-section. A ring-shaped encoder body 109 is adhered to the side of the annular supporting member 108 along its whole or entire periphery. The encoder body 109 has N and S poles alternately arranged along its circumference.
The inner end opening of the outer member 100 is closed by a cover 110. The cover 110 is formed of non-magnetic sheet material such as non-magnetic stainless steel sheet, aluminum alloy sheet or high functional polymer etc. The cover 110 has a dish-shaped configuration and includes a disc-shaped closing plate portion 111 and a cylindrical fitting portion 112 formed around a periphery of the closing plate portion 111.
The side face of the encoder body 109, forming the encoder 107, is arranged opposite to and close to the cover 110. The detecting portion 114 of the sensor 113 is arranged close to or abuts against the side of the cover 110. Thus, the detecting portion 114 is arranged opposite to and close to the encoder body 109 via the cover 110. Accordingly, the presence of the cover 110 prevents the entry of water, iron powder or magnetized debris etc. into the space between the sensor 113 and the encoder 107. Thus, this prevents damage to the sensor 113 and the encoder 107 as well as cyclic interfere or deterioration of the magnetic properties of the encoder body 109 (see e.g. Patent Document 1 mentioned below).
However, the prior art wheel bearing apparatus has several problems which are described below. First, since the cover 110 is secured to the outer member 100 simply by a metal-to-metal contact, it is impossible to have sufficient sealability in the fit portion without improving the surface accuracy and roughness of the fitting surfaces.
In addition, since the cover 110 is formed simply as an angular “C”-shaped cross-section, its rigidity is not sufficient. Thus, there is a risk that the cover would contact the encoder body 109 due to deformation of the cover 110 caused by impingement of pebbles, etc. Furthermore, since the detecting portion 114 of the sensor 113 opposes the encoder 107 via the cover 110, the detecting accuracy would deteriorate due to an increase of the air gap.
To solve these problems, a wheel bearing apparatus incorporating a rotational speed detecting apparatus with a structure shown in FIG. 25 has been proposed. This bearing apparatus is adapted to be secured on a knuckle (not shown). The bearing apparatus has an outer member 121, forming a stator member, an inner member 122, and double row balls 123, 123 contained between the outer and inner members 121, 122. The inner member 122 includes a wheel hub 125 and an inner ring 126 fit onto the wheel hub 125.
The outer member 121 has an integrally formed body mounting flange 121b on its outer circumference. The outer member inner circumference includes double row outer raceway surfaces 121a, 121a. The inner member 122 is formed with double row inner raceway surfaces 125a, 126a that oppose the outer raceway surfaces 121a, 121a of the outer member 121. One of the inner raceway surfaces 125a, 126a is formed on the outer circumference of the wheel hub 125. The other inner raceway surface 126a is formed on the outer circumference of the inner ring 126. The inner ring 126 is press-fit onto a cylindrical portion 125b, axially extending from the inner raceway surface 125a, of the wheel hub 125. Double row balls 123, 123 are contained between the outer and inner raceway surfaces. The balls 123, 123 are rollably held by cages 127, 127.
The wheel hub 125 is integrally formed with a wheel mounting flange 124 to mount a wheel (not shown). The inner ring 126 is axially immovably secured onto the wheel hub 125 by a caulked portion 128. The caulked portion 128 is formed by plastically deforming the end of the cylindrical portion 125b radially outward. A seal 129 and a cover (sensor cap) 133 are mounted on opposite ends of the outer member 121. The seals 129 and cover 133 prevent leakage of lubricating grease sealed within the bearing and the entry of rain water or dust from the outside into the bearing.
A magnetic encoder 130 is press-fit onto the outer circumference of the inner ring 126. The magnetic encoder 130 includes an annular supporting member 131 formed by a magnetic metal sheet. The sheet has a substantially L-shaped cross-section. An encoder body 132 is adhered to one side of the annular supporting member 131. The encoder body 132 is formed of a rubber permanent magnet mingled with ferritic powder and has N and S poles alternately arranged along its circumference.
The cover 133 is formed from synthetic resin as a cylinder capped 133b at one end. Its cylindrical portion 133a is press-fit into the end inner circumference of the inner-side of the outer member 121 to close the opening of the outer member 121 by the capped portion 133b. As clearly shown in FIG. 26, the cylindrical portion 133a is formed with a flange 134. The flange 134 is adapted to contact with the end face of the outer member 121. This makes it possible to exactly position the entire cover 133 axially relative to the outer member 121. Thus, this easily controls the position of the sensor 139 mounted on the cover 133.
The cap portion 133b of the cover 133 is formed with a cylindrical sensor mounting portion 135. An inserting portion 139a of the sensor 139 is inserted into a sensor mounting bore 136. The cover 133 is integrally molded with a metal core of a capped cylinder configuration in a region from the inner circumference of the cylindrical portion 133a to the inner surface of the cap portion 133b. The metal core 137 includes a cylindrical portion 137a and a cap portion 137b molded in the cylindrical portion 133a, 133b of the cover 133. The cap portion 137b forms a bottom portion of the cylindrical portion 137a. An opened portion of the sensor mounting bore 136, opposing the encoder body 132, is closed by the cap portion 137b of the metal core 137.
The metal core 137 is formed from a non-magnetic steel sheet with a thickness of about 0.3 mm. The presence of the cap portion 137b increases the strength of the cover 133. The non-magnetic property of the metal core 137 does not produce a negative influence on detecting accuracy of the rotational speed.
The sensor 139 is coated with a synthetic resin and mounted on the cover 133 by inserting the inserting portion 139a into the sensor mounting bore 136. The inserting portion 139a opposes the encoder body 132 via a predetermined axial gap. Sandwiched in the cap is the cap portion 137b of the metal core 137. A detecting portion (not shown) for detecting change of magnetic field generated by rotation of the magnetic encoder 130 opposes a portion to the encoder body 132. This detecting portion can output electric signals of the sensor 139 via an output cable 138.
As described above, since the opening portion of the sensor mounting bore 136 of the cover 133 opposing the encoder body 132 is perfectly closed by the cap portion 137b of the metal core 137 of non-magnetic steel sheet and having a capped-cylinder configuration, no foreign matter can enter into the inside of the wheel bearing apparatus. Thus, excellent sealability of the entire wheel bearing apparatus exists as compared with a through bore type sensor mounting bore which is not closed by any sealing member (see e.g. Patent Document Japanese Laid-open Patent Publication No. 2000-249138 and Japanese Patent No. 4286063).
In such a wheel bearing apparatus incorporating a rotational speed detecting apparatus of the prior art, since separation or small gaps tend to be caused, due to the difference in the liner expansion coefficients resulting from temperature variation caused by thermal impact, in the joined portion between the metal core 137 and the cover 133 of the synthetic resin, i.e. joined portions between the cylindrical portions 137a and 133a as well as joined portions between the cap portions 137b and 133b of the metal core 137 and the cover 133, it is difficult to maintain the initial sealability for a long term.